Focus Shift, Spherical Aberration, and the Canon 50L

The Canon 50mm f/1.2L is a remarkable lens. I won't go on and on about it since I'm not a lens expert. However, in my unqualified opinion, the 50/1.2L has to be one of the all-time great 50mm lenses in 35mm photography. The knock on the Canon 50L (besides it being somewhat "uncool" to be a Canon in a Canon-dominated world) is that it suffers a noticeable degree of focus shift. That is to say that at close focus distances, the focal plane at higher f-numbers (smaller apertures) is closer than the focal plane at f/1.2. Since the camera focuses each lens at full aperture, the result is that close range images at higher f-numbers end up out of focus (back focused). Apparently this degree of focus shift is the result of uncorrected spherical aberration. I assume the same uncorrected spherical aberration helps explain the excellent job this lens does with rendering out-of-focus background elements. With its modern C-Sonnar T* 1,5/50 ZM (the early version of which HCB used on his Leica M bodies), Zeiss describes focus shift more as a feature than a shortcoming. Click here to see a good demonstration of focus shift from the Sonnar. However, those who complain about the Canon 50mm f/1.2L suggest that Canon ought to have used a floating lens element to correct for near focus performance, just as they did with the 35mm f/1.4L and 85mm f/1.2L.

An alternate solution for the 50L focus shift - that Canon address it with a firmware update - was proposed some time ago by Joe Mama in the DPReview forums. At the time, I thought it was an excellent idea; but I wasn't convinced that it could be done. Lately I've been reading more about older film cameras and came across some interesting information about a relatively inexpensive autofocus film camera, the Konica Hexar AF ( The Hexar had a built-in 35mm f/2 lens that is said by some to have equalled or surpassed the best Leica had to offer at this focal length. Konica decided to leave a fair amount of uncorrected spherical aberration in this lens, which apparently contributed both to sharpness wide open and extremely good handling of bokeh at the expense of significant focus shift ( Their solution for the focus shift was to simply have the on-board computer adjust the AF in an aperture-specific manner.

Adding a floating lens element to correct for near focus would likely add to the cost, and potentially also the size, of the 50L. Seems to me that in this era of autofocus, the software route is even more elegant than the optical fix. Konica did this in 1993. Fifteen years later, I think Joe had it right. Canon should be able to manage this with a firmware update.

Posted by Amin

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